base: Simplify the definition of DTRACE.

Instead of gating the behavior of DTRACE based on TRACING_ON in the
preprocessor, move it to C++. Beyond being a little simpler, this
ensures that the value of Debug::x is always valid (the proper header is
included, x is spelled correctly, etc) even if TRACING_ON is false.

Change-Id: Ie0085c0f8753ad5283ef1850d493706b977c21a8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45006
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

src/base/debug.hh

@@ -151,11 +151,7 @@ void dumpDebugFlags(std::ostream &os=std::cout);
  * @ingroup api_trace
  * @{
  */
-#if TRACING_ON
-#   define DTRACE(x) (Debug::x)
-#else // !TRACING_ON
-#   define DTRACE(x) (false)
-#endif  // TRACING_ON
+#define DTRACE(x) (TRACING_ON && Debug::x)
 /** @} */ // end of api_trace
 
 #endif // __BASE_DEBUG_HH__